This invention relates generally to a liquid-operated booster for use in a brake system of a vehicle or the like so as to boost the operation of a master cylinder, and more particularly to the type of liquid-operated booster having a so-called jump-up function.
A liquid-operated booster of a general type is disclosed in U.S. Pat. No. 4,882,976. There are also known improved liquid-operated boosters, as disclosed in Japanese Laid-Open Patent Application Nos. 2-68258 and 3-143762, in which the pressure of a reaction force chamber is restrained at an initial stage of an inputting operation so as to reduce the initial operating force, thereby enhancing rise characteristics of a boosting force for a master cylinder. In the booster having such a jump-up function, the reaction force chamber is provided separately from a servo chamber, and a valve for achieving the jump-up effect is provided between the two chambers. This valve functions to prevent the introduction of the liquid pressure into the reaction force chamber until the liquid pressure introduced into the servo chamber reaches a predetermined level. The above Japanese Laid-Open Patent Application No. 3-143762 corresponds to U.S. patent application Ser. No. 07/691,645 (filed Apr. 25, 1991) and European Patent Application No. 91303809.7 (filed Apr. 26, 1991).
In the liquid-operated booster of this type, the servo chamber is formed at one end of a piston slidably received in a cylinder bore, and the reaction force chamber is formed between an input member, extending through the servo chamber into a recess of the piston, and the piston. Therefore, there is encountered a problem that when the input member and the piston move closer relative to each other in response to the operation of the brake, the volume of the reaction force chamber is reduced before the liquid pressure is introduced into the reaction force chamber, so that an internal pressure is produced in the reaction force chamber. As a result, the pressure of the reaction force chamber is increased, so that the pressure differential between the reaction force chamber and the servo chamber is small, and therefore a desired jump-up effect can not be obtained.